Geneticists are a step closer to understanding how plague evolved into one of the great scourges of human history. Yersinia pestis, the bacterium that causes bubonic plague, spreads through the bites of infected fleas. In the flea’s gut, plague bacteria multiply until a mass of bacteria blocks the passage to the flea’s stomach. The starving flea bites a host and feeds frantically, but since it can’t swallow its meal, it ends up regurgitating blood and bacteria back into its prey’s bloodstream, spreading the disease to a new host.
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